The traditional distribution grid is a passive grid without local energy sources and is fully supplied by the transmission grid. Due to increased industry focus on renewable sources small and medium size distributed energy resources, DER, appeared in the distribution grid. And their amount is expected to grow mainly driven by environmental and reliability concerns.
Distribution grids with a large amount of DER may also benefit from the adoption of normally closed loop/meshed topologies in opposition to the dominant radial scheme. These topologies will have positive effects on power losses, voltage regulation and network reliability. Finally with a large amount of controllable and coordinated DER a part of the distribution grid can intentionally disconnect from the main grid in emergency cases and continue operation in the islanded mode. This concept is known as a microgrid.
This evolutionary transformation makes distribution grids more active but it will also face many technical challenges. And one important aspect is protection of microgrids. The existing protection systems are designed for a traditional passive distribution grid with fixed settings and may act inadequately in constantly changing operating conditions of microgrid. This includes for instance changes in the magnitude and direction of short circuit currents, e.g. DER on/off, network configuration including islanding; reduction of fault detection sensitivity and speed in tapped DER connections; unnecessary tripping of utility breaker for faults in adjacent lines due to fault contribution of the DER; and Auto-reclosing of the utility line breaker policies may fail. The following proposals are developed with consideration of some of above aspects.
WO 2010/063816 describes a method and an apparatus for adapting at least one set of parameters to at least one Intelligent Electronic Device of an electrical power network having a plurality of switching devices. The method comprises the steps of: a) reading the current network status of the electrical power network, wherein the network status includes the status of the plurality of switching devices; b) simulating at least one network fault in the electrical power network; c) deducing at least one new set of parameters for the at least one Intelligent Electronic Device using a simulated fault current induced by the at least one simulated network fault under consideration of the current network status and the network topology of the electrical power network; d) setting the at least one set of parameters in at least one of the at least one Intelligent Electronic Device. A similar approach is described by Oudalov A., Fidigatti A., Adaptive Network Protection in Microgrids, Int. Journal of Distributed Energy Resources, Vol. 4, No. 3, pp. 201-225, 2009.
U.S. Pat. No. 7,525,782 describes an adaptive protection process for processing circuit breaker information that continuously monitors itself for changes so that as the state of the power distribution system changes. The adaptive protection algorithm traces flow of power through a circuit breaker system and assigns a value or rank to each circuit breaker based on its relative closeness to power sources and end loads. The rankings assigned to the breakers are used to determine the response time for each breaker to ensure system coordination and selectivity.
WO/2013/036385 describes a centralized coordination of setting and adjusting trip settings of electronic circuit breakers in an electrical distribution system by monitoring short circuit current availability, SCCA, and adjusting trip settings based on received SCCA estimates from SCCA monitoring devices installed at main, feeder, and branch nodes of the distribution system. The document concerns merely distribution system having multiple loads but not distributed energy resources. Thus, the changes of the short circuit levels are not that complex as in a distributed network having not only multiple loads but also distributed energy resource.